These comprise besides the flesh (muscles) of the different mammals, birds, and fishes several other portions of their bodies, as, for instance, various glands, brain, lung, liver, etc. Oysters and lobsters also belong to this group. In most instances the digestibility of this group of foods corresponds to their richness in fat. The less fat they contain the more digestible they are. Thus we have the following list of animal foods classified according to their digestibility:

The digestibility of food is greatly dependent on its quality and preparation. Young animals have soft and tender meat, whereas the flesh of old ones is tough. The different portions of the body vary also frequently in their digestibility. The time that has passed since the killing of the animal is also of importance. Fresh meat which is yet in its rigid state is tough and therefore very indigestible. In the preparation of the meat we must see that it is separated from all indigestible matter (fascia, tendons, cartilage). By pounding the meat the connective tissue surrounding the muscle fibre is torn. By chopping, scraping, or grinding the meat, its digestibility is increased. All other methods of preparing meat serve to improve its taste. For, according to Penzoldt, raw meat is more easily digested than that which has been boiled, broiled, or fried. The application of heat also diminishes the danger of infection, as many micro-organisms are destroyed by it.

Eggs are especially rich in albumin and fat. According to Penzoldt, soft-boiled eggs (three minutes in boiling water) are easiest to digest. Then come raw eggs and scrambled eggs, while hard-boiled eggs and omelet souffle are difficult of digestion. (Soft-boiled eggs remain in the stomach one and three-quarter hours, hard-boiled, three hours).

Milk is intended as the sole food of young animals and as such contains all the elements of a typical diet: (1) Albuminous substances in the form of casein and serum albumin; (2) fats in cream; (3) carbohydrates in the form of lactose or milk sugar; (4) salts, chiefly calcium phosphate; and (5) water. Milk does not stay in the stomach much longer than plain water and must therefore be considered very digestible.

Several articles of food are obtained from milk:

(a) Cheese, which is the casein precipitated with more or less fat, according as the cheese is made of skimmed milk (skim cheese), or fresh milk with its cream (Cheddar and Cheshire), or of fresh milk plus cream (Stilton and Double Gloucester). The precipitated casein is allowed to ripen, by which process some of the albumin is split up with formation of fat.

(b) Cream consists of the fatty globules encased in casein and which, being of lowest specific gravity, rise to the surface.

(c) Butter or the fatty matter deprived of its casein envelope by the process of churning.

(d) Buttermilk is the fluid obtained from cream after butter has been formed. It is therefore very rich in nitrogen.

(e) Whey is the fluid which remains after the precipitation of casein. It contains sugar, salt, and a small quantity of albumin.